Abstract [en]

Turbochargers are used on many automotive internal combustion engines to increase power density. The broad operating range of the engine also requires a wide range of the turbocharger compressor. At low mass flows, however, turbo compressor operation becomes unstable and eventually enters surge. Surge is characterized by large oscillations in mass flow and pressure. Due to the associated noise, control problems, and possibility of mechanical component damage, this has to be avoided.

Different indicators exist to classify compressor operation as stable or unstable on a gas stand. They are based on pressure oscillations, speed oscillations, or inlet temperature increase. In this thesis, a new stability indicator is proposed based on the Hurst exponent of the pressure signal. The Hurst exponent is a number between zero and one that describes what kind of long-term correlations are present in a time series.

Data from three cold gas stand experiments are analyzed using this criterion. Results show that the Hurst exponent of the compressor outlet pressure signal has good characteristics. Stable operation is being indicated by values larger than 0.5. As compressor operation moves towards the surge line, the Hurst exponent decreases towards zero. An additional distinction between the long-term correlations of small and large amplitude fluctuations by means of higher order Hurst exponents can be used as an early warning indicator.

Further tests using compressor housing accelerometers show that the Hurst exponent is not a good choice for real-time surge detection on the engine. Reasons are the long required sampling time compared to competing methods, and the fact that other periodically repeating oscillations lead to Hurst exponents close to zero independent of compressor operation.

Abstract [en]

Turbocharger compressors are limited in their operating range at low mass flows by compressor surge, thus restricting internal combustion engine operation at low engine speeds and high mean effective pressures. Since the exact location of the surge line in the compressor map depends on the whole gas exchange system, a safety margin towards surge must be provided. Accurate early surge detection could reduce this margin. During surge, the compressor outlet pressure fluctuates periodically. The Hurst exponent of the compressor outlet pressure is applied in this paper as an indicator to evaluate how close to the surge limit the compressor operates. It is a measure of the time-series memory that approaches zero for anti-persistence of the time series. That is, a Hurst exponent close to zero means a high statistical preference that a high value is followed by a low value, as during surge. Maps of a passenger-car sized turbocharger compressor with inlet geometries that result in different surge lines are measured on a cold gas stand. It is demonstrated that the Hurst exponent in fact decreases as the compressor moves towards surge, and that a constant value of the Hurst exponent can be used as a threshold for stable operation. Transient pressure signals of the compressor entering surge are analyzed in order to evaluate the time lag until surge can be detected using the Hurst exponent. Two surge cycles are usually needed to detect unstable operation. However, since the amplitude of these oscillations is relatively small for the first cycles, detection is possible before the oscillations grow into deep surge.

Abstract [en]

The compressor surge line of automotive turbochargers can limit the low-end torque of an engine. In order to determine how close the compressor operates to its surge limit, the Hurst exponent of the pressure signal has recently been proposed as a criterion. The Hurst exponent quantifies the fractal properties of a time series and its long-term memory. This paper evaluates the outcome of applying Hurst exponent based criterion on time-resolved pressure signals, measured simultaneously at different locations in the compression system. Experiments were performed using a truck-sized turbocharger on a cold gas stand at the University of Cincinnati. The pressure sensors were flush-mounted at different circumferential positions at the inlet of the compressor, in the diffuser and volute, as well as downstream of the compressor. Results show that the previously identified threshold value distinguishing between surge and stable operation when the analysis was carried out for a different and smaller compressor can be used also for this much larger compressor. The investigation concerning the sensor locations reveals that pressure sensors at the outlet or shortly upstream the volute tongue give the clearest distinction between fully stable operation and operation close to the surge line. Further investigations show that as currently implemented, the criterion would need a minimum sampling duration of 500 ms and sampling frequency of 512 Hz. An extended algorithm based on distinguishing between a mono- and multifractal pressure signal is shown to have potential as an early warning indicator.

Åbom, Mats

Abstract [en]

We investigate the generalized Hurst exponent, a measure of signal fractality, as an indicator of compression system stability. Tests were run on a centrifugal compressor of a light duty turbocharger on two test rigs: an acoustic test rig and a cold gas stand. While the compressor type is the same on both test benches, the other components of the compression system differ significantly, including the Greitzer B value and the presence of silencers. The Hurst exponent can be used to distinguish between stable and near-surge operation of the compressor independent of the compression system, with values larger than 0.5 at stable operation and values below 0.2 at the surge line. By extending the analysis towards a general Hurst exponent, we can identify a precursor to compressor surge that is also valid for both systems, namely the switch from monofractality to multifractality of the compressor outlet pressure signal. At low compressor speeds, this switch occurs at on the negative slope section of the compressor characteristic, where the system is theoretically fully stable. At higher compressor speeds, it coincides with the switch from negative to zero-slope, where theoretical models also predict compression system instability.

Mihaescu, Mihai

Abstract [en]

On-engine surge detection could help in reducing the safety margintowards surge, thus allowing higher boosting pressures and ultimatelylow-end torque. In this paper, experimental data from a truckturbocharger compressor mounted on the engine is investigated. Ashort period of compressor surge is provoked through a sudden, largedrop in engine load. The compressor housing is equipped with knockaccelerometers. Different signal treatments are evaluated for theirsuitability with respect to on-engine surge detection: the signal rootmean square, the power spectral density in the surge frequency band,the recently proposed Hurst exponent, and a closely related conceptoptimized to detect changes in the underlying scaling behavior of thesignal. For validation purposes, a visual observation of the air filtervibrations are also used to diagnose surge. The four signal treatmentsare compared with respect to their reliability as surge indicator andthe time delay between surge onset and indication. Results show thatthe signal power in the surge frequency band has reasonably goodproperties as surge indicator. The normal Hurst exponent isproblematic, since periodic vibrations from engine firing dominatethe scaling behavior. Root mean square and the above mentionedscaling exponent do not measure vibrations caused by surge directly,but rather the reduction in housing vibrations due to the engine loaddrop; nevertheless, it was found to be possible to design an indicatorthat gives good results based on the change in scaling behavior.

Abstract [en]

This paper experimentally investigates the effects of an upstream bended pipe on the compressor speedline slopes and surge line. Different orientation angles for the incoming bended pipe relative to the compressor scroll are investigated. The tests were carried out on a cold gas stand on a passenger car sized turbocharger. A bended pipe upstream of the compressor leads to an increase of the surge margin. This effect does not depend on the orientation of the bend. Comparisons with a straight inlet with artificially generated pressure losses indicate that the increase in operating range is an effect of the pressure losses generated in the bend.

Keywords

Turbochargers, Centrifugal Compressors, Compressor Surge

National Category

Vehicle Engineering

Identifiers

urn:nbn:se:kth:diva-191296 (URN)978-0-9572374-7-6 (ISBN)

External cooperation:

Conference

The 12th International Conference on Turbochargers and Turbocharging, London, UK, 17-18 May, 2016

Abstract [en]

Compressor performance prediction models, based on integral conservation of mass, momentum and energy with empirical loss terms, are important tools in early design stages. Two such models from literature are compared to numerical results for an automotive turbocharger radial compressor with a vaneless diffuser and a volute. Results show that these models are less accurate than fully three-dimensional numerical RANS CFD calculations at low impeller speeds and choke, but can compete at high impeller speeds. Of the two impeller models, one gives a more accurate prediction than the other. The diffuser and volute models investigated here show large differences to the CFD calculations at off-design conditions. A comparison of the impeller loss terms to CFD entropy increase indicates also possibilities for improvement in the impeller models.

Abstract [en]

Zero-dimensional (OD) compressor performance models, which consist of several sub-models for different loss terms, are useful tools in early design stages. In this paper, one typical model for centrifugal compressors is evaluated by comparing the loss-terms predicted by the model to data extracted from experimentally validated Large-Eddy-Simulation. The simulations were run on a truck-sized turbocharger compressor with a ported shroud and a vaneless diffuser. Four operating points are considered: One mass flow at design conditions and one mass flow close to surge, on two speedlines. The performance prediction models evaluated are impeller incidence loss, impeller skin friction loss, diffuser skin friction loss, and the tip clearance loss. Results show that the total losses are well-predicted by the model at design conditions. Friction losses are approximately independent of mass flow in the LES data, while the OD model assumes a quadratic increase. The assumption of constant tip clearance loss is validated by the LES data, and the impeller incidence loss model also fits the data well. Due to the ported shroud, most of the losses as calculated by entropy increase occur through isobaric mixing at the impeller inlet.

Sanz, Sergio

Mihaescu, Mihai

Abstract [en]

1D performance prediction modeling and steady-state CFD are applied to assess a high-performance centrifugal compressor. Computed total pressure ratio is compared with experimental data obtained from a gas stand. The focus of the paper is to assess the validity range of the methodologies used. Another aim is to quantify the relative differences between experimental and predicted data, and distinguish differences in the conjectured loss budget. The RANS data manifest overall a higher degree of accuracy than the 1D model when compared with experiments. The 1D model considered shows comparable accuracy at design condition but larger discrepancies at higher speedlines towards surge and choke. Component-wise parametric losses are correlated to pinpoint flow regimes with larger differences between 1D and RANS data. The result exposes significant disparity in the, impeller, vaneless diffuser and the volute model, respectively, especially off-design. Improving these features in the 1D modelling would potentially be profitable for improved accuracy in the performance prognosis.